There are more and more hybrid automobiles and working machines used in construction work in order to address energy saving and environmental issues. In hybrid automobiles, an electric motor for traveling is driven with electric power from an electric power accumulator, and the electric power accumulator is charged with electricity generated by the power of an engine. In hybrid construction machines, an engine is assisted by having a motor generator driven with electric power from an electric power accumulator, and the motor generator is driven by the power of the engine to generate electricity to charge the electric power accumulator.
Since the electric power accumulator is repeatedly charged and discharged, the degradation of the electric power accumulator progresses as the electric power accumulator is used for a longer period of time. In the case of using a capacitor as the electric power accumulator, the progress of the degradation of the capacitor increases the internal resistance and the amount of heat generated, so that in the worst-case scenario, the electrodes may be short-circuited. If the electrodes of the capacitor are short-circuited, fusion cutting occurs so that the capacitor becomes unusable.
Further, the degradation of the capacitor is caused by long-term use with repeated charge and discharge, overcharge, overdischarge, or heat generation. The state of degradation of the capacitor may be determined by measuring the internal resistance of the capacitor. (See, for example, Patent Document 1.)